Wire electrodischarge machining (EDM) is widely used to manufacture molds and other metal objects because of its capability to cut arbitrary, precise shapes in a highly automated manner under numerical control. The cutting is effected by electrical discharge between a wire electrode and workpiece. The process normally begins with insertion of the wire electrode through a starting hole in the workpiece. Means of inserting the wire automatically are essential, so that multiple cuts can be made in the same workpiece without human intervention.
Electrodischarge machines are therefore equipped with automatic wire feeding apparatus capable of inserting and feeding the wire automatically. Such apparatus generally comprises a pair of wire guides disposed above and below the workpiece for holding the wire electrode steady as the workpiece is moved for cutting, a motor-driven feed roller for feeding the wire electrode to one of the wire guides, a wire collection roller, also motor-driven, for receiving the wire electrode from the other wire guide and maintaining the necessary wire tension, and pipe guides for guiding the wire electrode between the rollers and the wire guides. Prior to insertion, the wire electrode is cut at a point between the feed roller and the first wire guide. After the workpiece is mounted with its starting hole positioned between the wire guides, the wire electrode is fed forward through a pipe guide to the first wire guide, then through the starting hole and out through the second wire guide to the wire collection roller. This completes the insertion operation, after which cutting can begin.
Until the tip of the wire electrode engages the wire collection roller, the wire electrode moves in an incompletely constrained manner, guided largely by its own stiffness. If the wire electrode is not perfectly straight, or if one of the wire guides is off-center, or if the workpiece is misaligned, the tip of the wire electrode may catch on the wire guide or workpiece, impeding further insertion. Insertion may also be impeded by friction between the wire electrode and wire guides. As the feed roller continues to turn, the impeded wire electrode is forced to bend or loop along its path until eventually it rides off the feed roller, at which point the automatic wire feeding apparatus stops functioning and human intervention is required to recover normal operation. This problem is especially common with wire electrodes having a low stiffness coefficient.